CAUSAL MECHANISMS FOR OCEANIC ANOXIC EVENTS I: THE ROLE OF IRON?

Iron “fertilization” is an important control on primary production in large areas of the modern ocean, and has been proposed as a mechanism for enhanced organic matter burial in marine sediments during Cretaceous Oceanic Anoxic Events. However, the singular view of iron as a control on primary production overlooks its role in sedimentary diagenesis, a factor that must be evaluated when considering organic matter accumulation. This study examines the diagenetic role of iron as a buffer of pore water sulfide, and its implications for bioturbation depth, oxygen exposure time, phosphorous cycling, and organic matter remineralization within surficial marine sediments. Biogeochemical model experiments indicate that an increase in iron delivery can serve to decrease organic matter burial (even under high rates of primary production) as a consequence of increased pore water sulfide buffering capacity. Alternatively, the experiments indicate that low concentrations of iron can facilitate enhanced organic matter burial by enabling sulfide buildup within pore waters. These model results suggest that iron in marine sediments can function as a “bottom-up” biogeochemical control on organic matter burial that does not require “top-down” oceanographic-forced changes in water column stratification or production. More generally, this “bottom-up” control on organic matter burial can be initiated by changes in the concentration of reactive iron (driving sulfide removal) relative to the concentration of labile organic matter (driving sulfide production), and a number of positive feedback mechanisms serve to amplify the response of organic matter burial to initial changes in these parameters. Comparison of model results with proxy data from Oceanic Anoxic Event II (middle Cretaceous) illustrates the complex role of iron as a control on both organic matter production and preservation.